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Flexible diamond Disk Polishing

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Diamond disk polishing with hand-held grinders offers the following advantages: 1) a major reduction in polishing time compared to traditional abrasives, 2) a cleaner shop environment without the dry grinding dust, 3) less worker fatigue with lighter tools requiring less pressure, and most importantly 4) an increased production throughput.

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Flexible diamond Disk Polishing

  1. 1. POLISHING STONE WITH DIAMONDS: DISK POLISHING problem, thanks to hook-and-loop (e.g., Velcro) and snail- lock fasteners. In fact, it takes half the time to moveConclusion through seven grit changes with diamond disks as it doesDiamond disk polishing with hand-held grinders offers the to move through four grit steps with silicon carbide. Snail-following advantages: 1) a major reduction in polishing time lock fasteners, may cost more that hook-and-loop, yet offercompared to traditional abrasives, 2) a cleaner shop envi- much greater durability and rigidity for a perfectly flat pol-ronment without the dry grinding dust, 3) less worker fa- ished surface.tigue with lighter tools requiring less pressure, and most Today, an experienced craftsman can cut his polishing timeimportantly 4) an increased production throughput. in half with diamond disks. Or, stated differently, an experi- enced craftsman can do twice the work using diamond pol-New technologies are not easily accepted within the stone ishing tools as he can using silicon carbide. And, if that 6"trade. For example, if everyone agrees on the superiority of by 2 monument top happens to be serpentine, it’s no bigdiamonds for cutting marble and granite, there still remains problem for diamond tools. Whereas for conventional sili-some controversy regarding the superiority of diamonds for con carbide abrasives, it would very difficult.performing the polishing operation, despite the fact thatboth operations have strong similarities. Indeed, many of Todaythe same people who would never go back to silicon car- The use of diamond disks for polishing edges is now wide-bide saw blades readily continue to use silicon carbide spread throughout the stone fabricating industry in the U.S.abrasives for finishing. It is observed during a typical StonExpo trade fair that 90% or more of the exhibiting fabricators were using diamondA bit of history disks in some part of their fabrication sequence. EvenTraditionally, polishing was done using silicon carbide those from overseas where labor costs are much less.wheels in three or four grit sizes. The work was physicallydemanding because the equipment was heavy. A right- Orange peel effectangle grinder with a silicon carbide cupwheel weighs as Almost all diamond disks in use today for polishing stonemuch as 15 pounds additionally, considerable pressures consist of diamonds embedded in a resin (plastic) matrix.are required to perform the required working a reasonable Diamonds are mixed with the resin to form blocks ratherlength of time for a modern economy. The silicon carbide like the lug pattern on the soles of shoes. The diamondspolishing was followed by an equally strenuous buffing op- are randomly scattered throughout the resin matrix to aeration. Finishing a 6" by 2 flat monument top, for example, specified depth. As the resin matrix is worn away, the dia-took an experienced craftsman up to an hour and a half. monds are gradually exposed. When diamond polishing was first introduced, the coarsestIn 1987, an innovative North American company introduced grit sizes disks were not resin bonded, but rather electro-diamond polishing tools for the stone fabricating shop. The plated. In electroplated disks, a single layer of diamonds iscompany offered a complete line of hand-held polishing attached to the surface of a disk by encapsulating (plating)disks that could achieve a finish in every way comparable it with nickel. The usual pattern is a series of small islandsto that produced by large automated machinery. Those first of diamonds. All the diamonds on the disk are exposed,diamond disks not only had to produce a finish that and all work simultaneously.matched the stringent quality standards of the industry, Unfortunately, electroplated disks produce random and rel-they had to provide benefits that outweighed a purchase atively deep scratches, something that resin bonded disksprice that was considerably above that of comparable sili- don’t do. This is because an electroplated disk is rigid on acon carbide tools. micro scale and a diamond particle that sits a little higher than the others, which always occurs will leave a deepThe problem of gaining acceptance was aggravated by the scratch. With resin bonded disks, the resin matrix is rela-fact that the polishing techniques that worked with silicon tively flexible, tending to cushion the diamond particles. Al-carbide were actually detrimental when using the diamond so, as the matrix is worn away exposing more and more ofdisks. Craftsmen, used to pushing hard with silicon carbide; a given particle, the tendency is for the particle to break outhad to be retrained to use diamond disks which worked rather and dig deeper into the stone until it is expelled,best with very little pressure. Grinders for silicon carbide causing even larger scratching.were geared for power whereas speed was needed for op-timum performance from the diamond disks. Polishing with Yet, there is another, perhaps more important, reason forsilicon carbide was done dry compared to diamonds tools the switch from electroplated to resin bonded disks, andwhich work best with a constant flow of water. Polishing that was because of the orange peel effect left by electro-with silicon carbide was performed using a sequence of a plated tools when processing granite. Granite is composedfew, typically three or four grits; and diamond polishing of two kinds of particles, hard quartz and feldspars whichusually required a minimum of seven. gives granite much of its durable characteristics and softerAll these factors made it difficult even to perform a suc- biotites, the components that give granite its color. Aggres-cessful demonstration or penetrate the market with ease. sive electroplated diamond disks remove material very rap-With time, though, the obstacles were overcome one by idly, producing a similarly aggressive water slurry. The ac-one. tion of the slurry tends to gouge the softer biotite, leaving aNow, right-angle grinders weigh less than five pounds, run pitted surface caused by minute depressions between thefast, and feature water supply connections are readily hard particles of granite. The open look of the orange peel-available. And the number of grit changes is no longer a like surface is accentuated as the polishing and buffing
  2. 2. progress. The harder granite particles take a high gloss be wavy. Moreover, the flexible disk will not do a good jobwhile the surfaces of the lower biotite particles remain in a of levelling untrue surfaces.no gloss state. The difference between both areas yield an Rigid disks, on the other hand, will do a better job of level-unacceptable finish known as “orange peel” ling and have less tendency to dig into soft areas. The rigidThis orange peel effect could eventually be alleviated by an disks are also less forgiving; the slightest inconsistency inintense and time consuming buffing procedure, as eventu- the polishing procedure will show up as a flaw when theally, the buffing media penetrates the small craters and work is completed. The novice can usually obtain better re-buffs the biotite. Unfortunately, though, the time saved by sults with a flexible disk. The experienced craftsman willusing diamonds for the polishing was largely lost during the generally prefer the rigid setup. Rigidity prevents roundingextended buffing operation. of the edges and produces a flatter desirable surface. In all cases, a slight cushioning is necessary to prevent rapidMarble, because of its more homogeneous structure, is not wear of the diamonds due to vibrations generated duringadversely affected by the aggressivity of the electroplated the rapidly rotating polishing sequence.disk or the ensuing abrasive slurry. It is simply ground fast-er. Hence, all metal-bonded diamond disks are safe to use Waterwith almost any type of marble, including limestones and Water is a desirable component of diamond polishing,sandstones. since it efficiently cools both the work and the disk. Resin bonds especially are quite intolerant of heat build-up. Be-Disk use yond its cooling properties, water flowing over the work at aA wide variety of diamond disks is available today. Popular steady rate yields other advantages; the job will go fastersizes for the fabrication shop are 3", 4", and 5" diameters, with less effort because the slurry helps remove material;where 4” is by far the most popular and larger and smaller there won’t be any dust; and the finished surface will besizes are typically reserved for specialty uses. superior to anything that can be done dry. This is just asThe most popular grit sizes for electroplated disks are #70, true for hand-held grinders as it is for large production ma-#120, #220, and #400. Beyond that, there is ample demand chines. Properly insulated electric tools incorporating waterfor both the coarser #30 and #50 grits and well as the much feeds are readily available, yet there remains a demand forfiner #600, especially for marble work. diamond disks that operate fully dry. These dry tools may still be useful, but using diamonds in dry polishing opera-Grit sizes for resin bonded disks commonly range from the tions is a compromise at best.coarsest #30 to a very fine #8000. Typically, #50, #120,#220, #400, #800, #1800, and #3000 are most commonly Rotation speedsused in the shop for processing granite surfaces. Marble For diamond abrasives, the best speeds are typically 3000-processing is less stringent and the combination of grits 7000 RPM for a 4" disk. Higher speeds allow the diamonds#50, #220, and #800 usually yields the desired finish. to do the work rather than relying on pressure. In fact, ex- cessive pressure will noticeably shorten the life of a dia-A point has to be made on the abrasiveness of the stone it- mond disk.self. Marble is soft, but tends to yield a more abrasive slurrythan granite, which is harder but less abrasive in slurry Break-inform. Some shops try to polish both types of stone with a All resin-bonded disks need to be broken-in, a process thatsingle set of disks, and this is a mistake. Disks are recom- exposes the diamonds enclosed in the plastic matrix on themended for specific materials based on grit size, plus the surface of the disks. This is easily done for all grit sizes onbond (electroplate, metal, or resin), the number of dia- the rough surface of the material being worked. No problemmonds that are working at any given time, and the hard- with the coarse grits. Just attach the disk and start grinding.ness and abrasiveness of the material. Some disks work For the finer grits, e.g., #3000, break-in is best done on awell with marble and poorly with granite, just as a marble surface that would be produced by a coarse grit of #120 ordiamond saw works well on marble and inadequately on so.granite. There are no universal disks any more than there A #3500 disk can be broken-in during the normal polishingare universal saws. routine, i.e., on a surface produced by a #1800 disk, but it will take some time. A disk is broken-in when all the grind-Flexibility vs. rigidity ing surfaces have lost the gloss they had when they cameThere is a trade-off between the flexibility and the rigidity of out of the package.diamond polishing disks. Flexibility is usually achieved bymeans of a rubber universal joint mounted on the spindle of Costsa right-angle grinder or by using a flexible disk attached On the basis of materials only, diamond polishing tools arewith a hook and-loop fastener. Rigidity is the result of a rig- significantly more expensive than silicon carbide. Theid disk attached with a snail-lock fastener. A good compro- overall advantages, however, clearly favors diamonds—mise consists of a rigid disk and a hook-and-loop fastener. less grinding time, improved production rates, better fin-Flexible disks tend to conform to the surface of the materi- ished surfaces, longer disk life—and add up to improvedal, polishing high and low areas with equal ease. In heavily profitability.veined marble or on extensive granite surfaces, the flexibledisk will tend to ride over the hard areas and dig into the For disk polishing with hand-held grinders, the advantagessofter portions. The job may go quickly, but the finish will of diamonds include: • A 50% reduction in polishing time.
  3. 3. • A cleaner shop without the dry grinding dust.• Reduced worker fatigue with lighter tools requiring lesspressure.• Increased production throughput.

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